The graphics don't really matter to me. But if you want to argue about graphics, I'd probably take Guitar Hero's. I don't like that grainy look they use on Rock Band constantly. The characters are also very cartoonish in Rock Band but they use more realistic venues. It looks stupid. Atleast GH goes with cartoonish people and cartoonish venues.
Rock Band's songs on guitar are way too easy. It's constant strings of the same notes. It gets very boring, very fast. The only thing great about Rock Band is that they introduced the modern drums to video games. I also prefer GH's drums. Also feel more realistic. (I don't have the GH set though... I'm using the Rock Band one.)